Langeled: Pipe Capacity Vs

Langeled: Pipe Capacity vs. Wall Thickness Selection

R. Bruschi, L. Vitali, E. Torselletti and D. Zenobi Snamprogetti SpA, Dept., Fano (PU), Italy A. Johannessen and R. Holme Statoil ASA, Stavanger, Norway

The Langeled Pipeline South (LPS) is characterized by a large diameter and thin steel wall, giving rise to an outer diameter-to-thickness ratio, D/t, larger than 45. The local buckling design equation in DNV OS F101 is explicitly deﬁned as applicable to a pipe D/t ratio equal to or lower that 45. It was then necessary to investigate if DNV OS F101 could also be made applicable as a project guideline for D/t > 45. The scope of this paper is to present the results of a desk study carried out during the project development; the study aimed to verify the applicability of the local buckling design equations enclosed in DNV OS F101. In particular, the following aspects are discussed in the paper: Finite Element Modeling of the pipe section of the LPS, including weld misalignment and the speciﬁed geometric tolerances; 3-dimensional FE Models aiming to analyze the LPS under relevant load conditions; FE calculations and sensitivity analysis aiming to determine the nonlinear loading curve (moment vs. curvature under axial/pressure loads), and to identify the limit bending pipe capacity and the relevant deformations; comparison of the calculated limit bending capacity with the results from the application of the DNV OS F101 design format.

INTRODUCTION • To brieflydescribe the FE model undertaken to analyzethe bending capacityof the LPS pipeline under relevant load condi- Ormen Lange is a large gas field 120 km off the west coast of tions. central Norway. The field has been developed with 2 subsea tem- • To show the results of an FE model studycarried out to ana- plates and two 3-in flowlines to shore at Nyhamna, where a gas lyze the effect of the relevant parameters, i.e. load combination, treatment facilityis being built. Norsk Hydrois the development pipe geometric characteristics (outer D/t ratio), girth weld param- operator while Norske Shell will operate the field and the plant. eters (mechanical strength, pipe joint misalignment), etc. As part of the Ormen Lange development, a 1200-km- • To compare the limit bending capacity/strain as obtained with long, large-diameter pipeline, Langeled, has been installed from the FE model with the limit bending capacity/strain as predicted, Nyhamna to Easington on the east coast of England. Statoil has using DNV OS F101 design formats. managed the Langeled Development Project on behalf of Norsk A commercial computer code environment was used to develop Hydro. Gassco, as the operator of all gas export pipelines from the the 3-Dimensional Finite Element (FE) Model (Hibbit, 2000). Norwegian Continental Shelf, will be the operator of the pipeline. Langeled is tied into the Sleipner field about midwayto Eng- Background of Local Buckling Mechanism land. The northern leg, from Nyhamna to Sleipner, is a 4-in- Nonlinear collapse/local buckling refers to the decrease in the diameter 250/215-bar pipeline, while the southern leg from Sleip- stiffness of a structure as the load increases. At the collapse ner to Easington is a 44-in-diameter 157-bar pipeline. (or buckling) load, the load deflection curve has zero slope and The Langeled Pipeline South (LPS) is characterised bya large the structure becomes unstable if the deformation is load con- diameter and thin steel wall, giving rise to an outer diameter- trolled. The load-deflection curve is called the primaryor funda- to thickness-ratio, D/t, greater than 45. This followed from the mental path; the deformation on the primarypath, the primary selection of the SAWL485 steel grade, which has allowed a mean- deformation. ingful reduction of the investment costs when market prices for A primarypath described bya nonlinear or linear stress analysis line pipe material had increased tremendously. of a so-called perfect structure maypossess forks or bifurcation The local buckling design equation in DNV OS F101 is explic- points. At the first bifurcation point, a new deformation pattern itlydefined as applicable to a pipe D/t ratio equal or lower than unexpectedlydifferent from the primarydeformation pattern may 45. This applies to both internal pressure and external pressure begin to develop. This is not because of a change in the external combined with bending moment and axial force. A desk studywas load, but because the equilibrium configuration on the primary then carried out in order to assess the strength and deformation path is unstable at this point. The resulting deformation pattern is capacityof the LPS pipeline, including the bifurcation buckling. called the bifurcation mode; the associated load deflection curve, The scope of this paper is: the secondarypath. • To discuss the analysis methodology and the results of the Real structures are imperfect. There are unavoidable minor bifurcation buckling analysis. irregularities in dimensions and material properties (submarine pipelines are ovalised and present thickness variations, weld mis- alignments, etc.). The imperfections maytrigger a primarydefor- Received January3, 2008; revised manuscript received bythe editors mation pattern different from the theoretical one. The bifurcation March 20, 2008. The original version (prior to the final revised concept maythen be without meaning for real structures. Depend- manuscript) was presented at the 17th International Offshore and Polar Engineering Conference (ISOPE-2007), Lisbon, July1–6, 2007. ing on pipe geometry, mainly outer D/t ratio, and load condition KEY WORDS: Submarine pipeline, local buckling, bifurcation buckling, (presence of internal pressure or lack of presence), bifurcation limit state, failure mode. buckling mayor maynot predict the limit load.